Oil country tubular goods, which are steel pipes used in the excavation of oil wells and gas wells and which includes tubing through which oil or gas flows and casing which surrounds tubing in a well and has a larger diameter than tubing, normally have an overall length exceeding 10 meters. In the field, a large number of oil country tubular goods are successively connected together by threaded joints to a length sufficient for the resulting string of oil country tubular goods to reach an underground oil or gas layer. The depth of an oil or gas well is often from several thousand to ten thousand or more meters, so the number of oil country tubular goods which are connected in series by a threaded joint becomes a large number.
A threaded joint for connecting oil country tubular goods to each other is constituted by a pin having male or external threads and a box having female or internal threads. In its environment of use, threaded joints for oil country tubular goods are subjected not only to compound stresses comprising axial tensile stresses caused by the weight of the connected oil country tubular goods and the threaded joints themselves and stresses caused by internal and external pressures, but also to thermal stresses produced by geothermal heat. A threaded joint for oil country tubular goods needs to maintain gas tightness (sealing properties) without being damaged even under such a severe environment.
When oil country tubular goods are lowered into a well, a threaded joint which has once been connected is sometimes loosened and disconnected and then reconnected. Therefore, API (American Petroleum Institute) standards require that a threaded joint for oil country tubular goods be capable of being connected and disconnected 10 times for a joint for tubing and 3 times for a joint for casing without undergoing unrepairable seizing referred to a galling while maintaining gas tightness.
In order to meet this demand, in the past, galling resistance of threaded joints for oil country tubular goods (also referred to as OCTG connections) has been evaluated by a simulation prescribed by ISO 13679 in which short steel pipes having an overall length of around 1-1.5 meters are used as test pieces. The test pieces are subjected to repeated connection (also referred to as make-up) and disconnection (also referred to as break-out).
However, when a threaded joint for actual oil country tubular goods, which may have a length exceeding 10 meters, is connected or disconnected, the upper portion of the upper pipe of the two oil country tubular goods being connected gyrates due to bending which is invariably present in oil country tubular goods. As a result, a moment (load) acts on the threaded joint, and the threads of the joint may sometimes be damaged at an early stage of use. However, in a simulation using short steel pipes as test pieces, it is not possible to reproduce this type of damage to the threads of a joint, so it is not always possible to accurately evaluate the galling resistance of a threaded joint for oil country tubular goods with a length exceeding 10 meters.
In the past, tests for evaluating the galling resistance of a threaded joint using actual oil country tubular goods have been carried out in the field of operation or using a test apparatus which is roughly the same as an actual oil rig. For example, Patent Document 1 discloses an apparatus capable of moving an oil rig structure on an offshore platform. It is described therein that the galling resistance of a threaded joint for oil country tubular goods is evaluated using a test apparatus in which the rear end (upper end) of a first oil country tubular good which is positioned above is directly grasped so as to align this first tubular good with a second oil country tubular good which is positioned below in a fixed state and then rotate the first oil country tubular good in order to screw the first tubular good into the second tubular good.
As shown in FIG. 1 of Patent Document 1, the test apparatus disclosed in that document is extremely large-scale and tall. If it is attempted to evaluate the galling resistance of a threaded joint of an actual oil country tubular good using such a test apparatus which can be applied to practical use, the time required for preparation for the test, the number of testing personnel, and costs become extremely large, so it is not possible to easily and inexpensively evaluate the galling resistance of an actual threaded joint for oil country tubular goods. Accordingly, there is a strong demand by users of oil country tubular goods for a test apparatus which can accurately, easily, and inexpensively evaluate the galling resistance of a threaded joint for an oil country tubular good actually used in the field.
Thus, it is not always possible to accurately evaluate the joint performance (particularly the galling resistance) of a threaded joint for an actual oil country tubular good by a conventional simulation using a short steel pipe, and if it is attempted to directly evaluate the properties of actual oil country tubular goods, there are the problems that the time required for preparation of the test, the number of test personnel, the costs, and the like are enormously increased since there are a very wide variety of specifications of oil country tubular goods which differ with respect to dimensions and target performance.
In recent years, in offshore oil rigs which have come to be much used as the depth of wells has increased, although it depends upon the type, oil country tubular goods are connected by threaded joints in an environment which is always oscillating. Up to now, there was no test apparatus which could evaluate the galling resistance of a threaded joint for oil country tubular goods in an oscillating condition. Accordingly, there is a strong demand for such a test apparatus by users of oil country tubular goods.    Patent Document 1—U.S. Pat. No. 5,492,436